1. Field of the Invention
The present invention relates to a reading technique of an optical disc drive.
2. Description of the Related Art
Although the scope of application of the present invention is not limited to Blu-ray Discs (BDs), the following description is mainly based on the application to BD, and the terminology used hereinafter is basically the same as that used for BD.
Most current optical disc apparatuses such as BDs employ a high-frequency (HF) wave superimposition method to suppress noise generated by a laser diode, which is used as the light source thereof. This technique is disclosed in Non-Patent Document 1 and is well known for those skilled in the art. Therefore, the following description contains only essential features and not details.
When a laser beam reflected from a disc enters a laser diode being oscillated, the oscillation state becomes unstable, and thus producing significant laser noise. In order to avoid this, a high-frequency (HF) wave superimposition method is used. In this method, a high-frequency signal is superimposed on the drive signal of the laser diode and the laser is made to perform pulse emission. The emission waveform is a cyclic pulse as shown in FIG. 2. In this case, the laser pulse interval (modulation frequency) and the ratio of the emission period to this (duty) are the parameters that are adjusted to minimize the laser noise. That is, the frequency and the duty are determined so that a laser pulse reflected from a disc would not enter the laser diode being oscillated.
A pulsed laser beam is focused onto a recording layer of the optical disc. Then, the amplitude of the laser pulse is modulated because the intensity of the reflected laser beam differs depending on whether the portion irradiated with the laser beam is a mark or space. Provided that the bandwidth limitation due to a photodiode for reading and a current to voltage converter amplifier is nil, the read signal waveform will be a shape like the one shown in FIG. 3. Hereinafter, a signal composed of such a read pulse train will be referred to as a pulsed read signal. The dashed line in FIG. 3 is the read signal waveform obtained when the laser is continuously oscillated at the same output as the peak of the laser pulse during high-frequency superimposition. That is, the shape of the upper envelope of the pulsed read signal is a read signal waveform based on a continuous light beam. Accordingly, by envelope detection, that is, by passing the pulsed read signal through a low-pass filter having a cut-off frequency that is sufficiently lower than the frequency of the superimposed high-frequency current, it is possible to obtain a desired read waveform. In current optical disc apparatuses, this is achieved by the bandwidth limitation of a system including a photodetector and a current to voltage converter amplifier, and an analog equalizer.
Pulsing a read signal is equal to a kind of amplitude modulation. Thus, a line-like spectrum of a superimposed high-frequency signal as well as modulated read signal components in the vicinity thereof is observed. Accordingly, in this specification, the superimposed high-frequency signal will be simply referred to as a carrier. To give an example of a carrier frequency, approximately 400 MHz is normally used in the case of BD. This is entirely determined according to the optical path length of the reading optical system, so it is thought that there is not a big difference between apparatuses.
FIG. 4 shows an example of a spectrum of a pulsed read signal. The dashed line in FIG. 4 schematically represents the bandwidth limitation by the system including the photodetector and the current to voltage converter amplifier, and the analog equalizer. That is, converting a pulsed read signal into a continuous signal with the aforementioned conventional method could result in the attenuation of all harmonic components. Thus, the amplitude of the obtained read signal becomes small and the ratio of the amplitude to the pulsed read signal amplitude is approximately equal to the pulse duty.
As a technique for improving the reduction of SNR resulting from a decrease in the obtained amplitude as described above, there is known MTD (Multi-tone demodulation). Such a technique is disclosed in detail in Patent Document 1, and is also described in Non-Patent Document 3. With MTD, it is possible not only to compensate for the read signal SNR but also to solve a problem that separation of a read signal and carrier becomes difficult when high-speed reading is performed as detailed in Patent Document 1. That is, a signal obtained by MTD fundamentally contains no line-like spectrum of carriers.
In recent years, it has been known by those skilled in the art that a digital scheme such as a PRML (partial response most-likely) method is the mainstream read signal processing of optical discs. In such a signal processing system, a PLL (phase locked loop) circuit for synchronizing the clock of a read signal with the clock of the signal processor is typically digitized. However, in practice, such a digitized PLL includes a plurality of analog components such as a voltage controlled oscillator and a DAC (digital to analog converter). The use of analog components is problematic in that the characteristics thereof can easily vary. Thus, in recent years, signal processing systems that include no analog components have been studied as described in Non-Patent Document 4.    [Patent Document 1] JP Patent Publication (Kokai) No. 2007-73147 A    [Non-Patent Document 1] “Optics” Vol. 14, No. 5, pp. 377-383    [Non-Patent Document 2] Frank Op't Eynde, Willy Sansen, “Analog Interfaces for Digital Signal Processing Systems”, Kluwer Academic Publishers, 1993 Boston/Dordrecht/London, pp. 91-92    [Non-Patent Document 3] Atsushi Kikukawa, Hiroyuki Minemura, “Novel HF-pulse read signal converter for increasing read signal SNR”, Digest of International Symposium on Optical Memory 2007, pp. 302-303    [Non-Patent Document 4] Floyd M. Gardner, “Interpolation in Digital Modems—Part I: Fundamentals”, IEEE Transactions on Communications, Vol. 41, pp. 501-507 (1993)